HaematologY. Dr Tom Bate Again!

Transcription

1 HaematologY Dr Tom Bate Again!

2 THE FRCA

3 What you need to know

4 Blood Groups ~30 blood group systems recognised (ISBT) ABO, Rhesus, Lewis, Kell, Duffy, Kidd MNS system, Kell system The ABO antigens are the most abundant on the red cell membrane and the most important in transfusion medicine Serum contains antibodies which are IgM molecules which rapidly agglutinate cells which contain the antigens they are directed against

5 Inheritance of Blood Groups Chromosome 9q34 3 alleles A, B, O A and B are co-dominant over O O produces no antigens and is recessive 4 phenotypes A (Genotypes AA, AO) B (Genotypes BB, BO) AB (Genotype AB) O (Genotype OO)

6 Blood Group (UK incidence) Antigens on RBC Antibodies in Serum A (42%) A Anti-B B (8%) B Anti-A AB (3%) Universal Recipient A & B Neither O (47%) Universal Donor Neither Anti-A & Anti-B

7 Rhesus Blood Group System Rhesus positive refers to cells that carry the D antigen Unlike ABO system there are no naturally occurring antibodies. Antibodies are only produced on exposure to the appropriate Rh antigens (i.e. blood transfusion or foeto-maternal haemorrhage) Inherited according to simple Mendelian genetics

8 AAGBI, g/dl Essential <7 Transfuse 8 10 Safe >10 No transfusion Symptomatic Transfuse

9 Haemoglobin Normal Haemoglobin Tetramer consisting of 4 globin chains, each combined with one haem moiety Each molecule contains 2α chains and 2 non-α chains

10 Haemoglobin RBC normally contain 3 types of Hb Hb-A (95%) α 2 β 2 Hb-A 2 ( %) α 2 δ 2 Hb-F (<2%) α 2 γ 2 : protects against SCD

11 Sickle Cell Disease Single base mutation: glutamine for valine at position 6 of the β globin chain HbS has normal O 2 carrying capacity but with deoxygenation: polymerises - crystal-like rods - inflexible sickle-shaped erythrocytes Haemolysis and organ damage HbAS; HbSS; HbSC/HbSβ-thal

12 Sickle Cell Disease Low Hb (6-9) Blood film: reticulocytes, sickle and target cells Screening tests Electrophoresis?Exchange transfusions (<30% HbS) AVOID Dehydration Hypoxia Hypothermia Acidosis PROVIDE Pain control (RA) Antibiotics

13 Thalassaemias Partial or complete failure of synthesis of one or more of the globin chains, with overproduction of the normal chains Defective haemoglobinisation of the RBC and haemolysis β-thal Major Severe anaemia due to haemolysis, iron overload β-thal Minor Clinically asymptomatic

14 The Bit You have all been waiting for! Why does a snowman not bleed to death when stabbed in the eye??

15 Haemostasis & Coagulation Overall haemostatic strategy: Constriction of damage vessel Plug the hole (platelets) Consolidate the plug (generation of thrombin) Dissolve the plug after healing

16 Vascular trauma Primary Haemostasis Vasoconstriction and exposure of subendothelium Platelet activation and adhesion to collagen Platelet shape change and granule release leading to local concentrations of: platelet activators coagulation factors vasoconstrictors Primary plug formed and blood loss stopped

17 Secondary Haemostasis The classical coagulation pathway The cell based model of coagulation

18 Secondary Haemostasis The classical coagulation pathway is a proteolytic cascade Each enzyme of the pathway is present in the plasma in its inactive form The end result of the clotting cascade is the production of thrombin (from prothrombin) for the conversion of fibrinogen to fibrin 3 phases: Intrinsic pathway Extrinsic pathway Final common pathway

19 Intrinsic Pathway Activated when blood comes into contact with sub-endothelial connective tissues (collagen) Quantitatively more important than extrinsic pathway, but slower

20 Intrinsic Pathway Ultimately activates factor X, which is the first molecule of the common pathway Factor X is activated by a complex of molecules containing factor IXa, factor VIII, calcium and phospholipid (from the platelet surface, where the reaction takes place) Precise role of factor VIII not understood, but essential (Haemophiliacs) Factor VIII is modified by thrombin, which greatly enhances factor VIII activity, promoting factor X activation

21 Extrinsic Pathway Provides a very rapid response to tissue injury, producing factor Xa almost instantaneously The main function of the extrinsic pathway is to augment the function of the intrinsic pathway Components unique to extrinsic pathway are tissue factor (or factor III) and factor VII

22 Extrinsic Pathway Tissue factor (FIII) is a transmembrane protein found in vesicles below the cell surface in most human cells. It is relocated to the surface after vascular damage where it binds rapidly to factor VII FVIIa forms a complex with FIII (TF), calcium and phospholipid which then activates FX

23 Common Pathway End result is the production of thrombin (from prothrombin) for the conversion of fibrinogen (FI) to fibrin Exposure of fibrinogen to thrombin results in rapid proteolysis of fibrinogen and the release of fibrinopeptide A A second peptide, fibrinopeptide B, is then cleaved by thrombin and the fibrin monomers formed polymerise spontaneously to form an insoluble gel

24 Common Pathway The polymerised fibrin, held together by noncovalent and electrostatic forces, is stabilised by factor XIIIa (produced by the action of thrombin on factor XIII) These insoluble fibrin molecules aggregate, together with aggregated platelets, block the damaged blood vessel, and prevent further bleeding

25 Coagulation Screening Intrinsic Pathway - Partial Thromboplastin Time (PTT) (25-35 secs) Extrinsic Pathway - Prothrombin Time (PT) (11-15 secs) Common Pathway - PTT & PT

26 Inhibitors of Coagulation Thrombin is a very potent enzyme and various inhibitory agents limit its function to prevent uncontrolled and widespread intravascular coagulation Antithrombin III and Protein C (& cofactor Protein S) Liver removes and degrades circulating activated clotting factors Heparin acts by binding to antithrombin III, greatly accelerating inactivation of FXa and thrombin

27 Fibrinolysis Once haemostasis is restored and the tissue is repaired, the clot must be removed by the fibrinolytic pathway The enzyme plasmin is the end product of this pathway Activators found in vascular endothelium, tissues and other sites convert the proenzyme plasminogen to the active enzyme plasmin Tissue plasminogen activator (tpa) is the most important activator Plasmin digests fibrin and fibrinogen to form Fibrin Degradation Products (FDPs)

28 The Fibrinolytic Pathway

29 Vitamin K Factors II, VII, IX, and X are structurally similar serine proteases Synthesised in the liver as inert precursors Converted to biologically functioning molecules by vitamin K dependent enzyme gammadecarboxylase Precursor Gammacarboxylase Functional clotting factor Reduced Vitamin K Oxidised Vitamin K Vitamin K Reductase WARFARIN

30 TEG - The Future

31 SO SIMPLE! OR IS IT?

32 WHAT SORT OF SAUSAGE! A: normal B: prolonged (anticoagulation and factor deficiency) C: decreased maximum amplitude (thrombocytopenia and platelet function inhibitors) D: fibrinolysis E: hypercoagulability F: disseminated intravascular coagulation (DIC) G: end-stage DIC (hypocoagulation)

33 The Cell Based Model Although the classical model of coagulation supports lab tests of coagulation disorders, it does not adequately explain the mechanisms leading to haemostasis in vivo A cell-based model of haemostasis has been developed to replace the classical model of the coagulation cascade This model suggests that haemostasis occurs on different cell surfaces in three overlapping steps: 1. Initiation 2. Amplification 3. Propagation

34 The Cell Based Model Initiation occurs on a tissue factor-bearing cell In the amplification phase, platelets and cofactors are activated to prepare for large-scale thrombin generation Finally, propagation occurs on the surface of platelets, and results in the propagation of large amounts of thrombin

35 1. Initiation phase Injury of vessels wall leads to contact between blood and subendothelial cells Tissue factor (TF) is exposed and binds to FVIIa or FVII which is subsequently converted to FVIIa The complex between TF and FVIIa activates FIX and FX FXa binds to FVa on the cell surface

36 2. Amplification phase The FXa/FVa complex converts small amounts of prothrombin into thrombin The small amount of thrombin generated activates FVIII, FV, FXI and platelets locally. FXIa converts FIX to FIXa Activated platelets bind FVa, FVIIIa and FIXa

37 3. Propagation phase The FVIIIa/FIXa complex activates FX on the surfaces of activated platelets FXa in association with FVa converts large amounts of prothrombin into thrombin creating a thrombin burst. The thrombin burst leads to the formation of a stable fibrin clot.

38 Haemostatic drug therapy Coagulation factor concentrates: beriplex Antifibrinolytics: tranexamic acid, aminocaprioc acid, aprotonin Desmopressin (Haemophilia A, vwd, uraemic patients) rfvii (Novoseven),?the ultimate haemostatic drug!!!!

39 PCC ( Beriplex ) II, VII, IX, X Admin over 5-10 mins Correct INR within 5 mins Elective: stop warfarin for 4 days 24hrs: PO vitamin K 6hrs: IV vitamin K Immediate: PCC 15-25u/kg

40 NovoSeven Mode of Action Eptacog alfa (activated) Tissue factor (TF)/FVIIa, or TF/rFVIIa interaction, is necessary to initiate haemostasis At pharmacological concentrations rfviia directly activates FX on the surface of locally activated platelets. This activation will initiate the thrombin burst independently of FVIII and FIX. This step is independent of TF. The thrombin burst leads to the formation of a stable clot

41 NovoSeven Presentation Uses Doses Precautions Powder for injection with solvent Patients with haemophilia and congenital FVII deficiency Haemophilia: 90 mcg/kg FVIIa deficiency: 15 30mcg/kg every 4 6hrs Close collaboration with haematologist Side-effects MI, arterial, venous thrombotic events Cost 1.2 mg 664, 2.4mg 1329, 4.8mg 2658

42 ANY QUESTIONS

43 THANK YOU SCTG primary FRCA OSCE/VIVA day Jan 7th 2013 Brighton